This invention relates to a plasma operation apparatus and more particularly to a plasma operation apparatus suitable for performing thin film deposition on a specimen (or substrate) surface or etching, sputtering or plasma oxidation for the specimen surface by utilizing plasma generated by microwave discharge.
The plasma operation apparatus utilizing plasma generated by microwave discharge prevailing in a magnetic field has, within a discharge tube (also called a plasma generation chamber) forming a part of discharge space, a position at which electron cyclotron resonance (ECR) is caused by the magnetic field and a microwave and has a magnetic flux density distribution which decreases from the ECR conditioning position toward a specimen stand disposed within a specimen chamber. Consequently, plasma generated near the resonance position is decreased in density by an order of one to two or more during its transport from the discharge tube to the specimen stand and the plasma operation can not be done with high efficiency.
FIG. 1 illustrates an apparatus disclosed in "CVD utilizing ECR plasma", Transactions of 31st Semiconductor Integrated Circuit Technology Symposium held on December 3 and 4, 1986, pp. 49-54 and referred to as prior art example A hereinafter.
In the prior art example A, when a microwave 4 is introduced, through a wave guide 3 and an incident window 5, into a discharge tube 2 surrounded by an external magnetic field coil 1 and the electron cyclotron motion in a magnetic field due to the magnetic coil 1 resonances with the microwave 4 at the resonance position, resonant electrons collide with and ionize a gas 6 for plasma, thus generating plasma. Under the influence of a magnetic field divergence, the generated plasma is then pushed out into a specimen chamber 9 coupled to the discharge tube 2 and housing a specimen stand 8 for carrying or holding a specimen 7. This plasma alone or along with atoms or molecules of a material gas 10 additionally introduced into the specimen chamber 9 and excited or ionized by that plasma is used for plasma operation of a surface of the specimen 7.
FIG. 2 shows a magnetic flux density distribution occurring between the microwave incident window 5 and the specimen stand 8, where a value along the axis ordinate represents the distance in the direction of the central axis measured from the origin located at the boundary between discharge tube 2 and specimen chamber 9 and abscissa axis represents the magnetic flux density. In the case of this prior art example A, the magnetic flux density Be effective to cause electron cyclotron resonance at a frequency of 2.45 GHz of the incident microwave 4 is 875 Gausses and FIG. 2 indicates that the resonance conditioning position is axially about 3 cm distant from the microwave incident window 5. Then, taking into account the characteristic of propagation of the microwave through the plasma and the resonance absorption condition for microwave energy, only a region inside the discharge tube 2 and which is within 3 cm distant from the microwave incident window 5 proves to be effective for plasma generation. Plasma generated in this region is transported over a distance of about 35 cm toward the specimen stand 8 under the influence of force due to the magnetic field divergence and of polarity diffusion. In this transport, the long transport distance and an abrupt decrease in the magnetic field (magnetic flux density) cause a loss and because of this loss, the density of plasma reaching the surface of the specimen 7 through transport tends to be smaller than that of plasma near the resonance position at which the electron cyclotron resonance occurs.
FIG. 3 illustrates another apparatus disclosed in "Films of a - Si:H prepared by ECR plasma enhanced CVD", Transactions of 31st Semiconductor Integrated Circuit Technology Symposium held on December 3 and 4, 1986, pp. 61-66 and referred to as prior art example B hereinafter, and FIG. 4 shows magnetic flux density distributions in the FIG. 3 apparatus. In FIG. 3, elements corresponding to those of FIG. 1 are designated by identical reference numerals. When compared to the magnetic flux density distribution of the prior art example A, the level of the magnetic flux density distributions shown in FIG. 4 is higher as a whole. It will also be seen from FIG. 4 that the position for 875-Gauss magnetic flux density corresponding to the ECR position still lies within the discharge tube 2 and magnetic flux density exceeding 875 Gausses also prevails within the discharge tube 2, indicating that a region effective for the resonance absorption of microwave measures about 2/3 of the discharge tube 2 at its maximum. In addition, the magnetic flux density is abruptly decreased toward the specimen stand 8. Consequently, as in the case of prior art example A, the density of plasma generated near the resonance position tends to suffer from a loss and decrease during diffusion of the plasma toward the surface of the specimen 7.
FIG. 5 illustrates still another apparatus disclosed in JP-A-59-3018 and referred to as a prior art example C hereinafter, and FIG. 6 shows a magnetic flux density distribution in the FIG. 5 apparatus. In FIG. 5, elements corresponding to those of FIG. 1 are designated by identical reference numerals. The prior art example C is directed to the configuration of a mirror magnetic field type frequently used in the plasma confinement method with the view of raising the plasma density and additionally has a complemental permanent magnet 13 for raising magnetic flux density near the surface of the specimen 7 housed in the specimen chamber 9. In this prior art example C, the incident microwave 4 propagates through a region designated at (I) in FIG. 6 in which the magnetic flux density is higher than that at the resonance position, and the microwave 4 then undergoes resonance absorption by the plasma near a first resonance position designated at s in FIG. 6. And, it is difficult for the microwave reaching the first resonance position to pass therethrough and propagate into a smaller magnetic flux density region designated at (II) in FIG. 6 because this tendency of the microwave is resisted by the plasma. If the propagation leaks for approaching a second resonance position designated at t in FIG. 6 which is near the specimen 7 on the specimen stand 8 and plasma is generated at the second resonance position, the plasma will be forced to direct toward the discharge tube owing to a magnetic field divergence appearing near the second resonance position, with the result that as in the case of prior art examples A and B, the density of plasma incident upon the specimen 7 tends to be smaller than that of the plasma near the first resonance position.
Disclosed in JP-A-56-155535 is still another apparatus wherein, as in the prior art example A, a plasma activated species is generated in a plasma generation chamber and a plasma flux stemming from the activated species under the application of a divergent magnetic field is bombarded for operation upon a substrate to be operated which is sufficiently distant from a region of the maximal production efficiency of activated species.
Further, a known plasma operation method as disclosed in JP-A-57-79621 intends to improve efficiency and employs a magnet disposed externally of a substrate operation chamber and which restricts the radius of plasma flux to raise plasma density.
All of the prior art described hereinbefore does not thoroughly consider the problem that the density of plasma generated by the microwave subject to the electron cyclotron resonance in the magnetic field suffers from a loss during transport of the plasma to the specimen surface, that is, the problem concerning life of the plasma activated species or deactivation thereof during transport of the plasma activated species to the substrate to be operated, and they can not always succeed in improving efficiency of the plasma operation. Also, in the prior art, excellent characteristics of the produced films e.g., densification, crystallinity and stoichiometry of the deposited films can not be obtained.
Another article relevant to the present invention is "Low Temperature Chemical Deposition Method Utilizing an Electron Cyclotron Resonance Plasma" by S. Matsuo and K. Kiuchi, Jpn. J. Appl. Phys. 22(4), L210, 1983.